The present invention relates to electrostatic methods and apparatus for forming fibers from fluids.
In conventional commercial production of low diameter fibers, a liquid material such as a liquid polymer is forced through a small orifice in an apparatus referred to as a spinneret. The liquid polymers utilized in many fibers are extremely viscous and difficult to pass through a small orifice. These methods encounter practical difficulties.
Certain methods of electrostatic formation of fibers from liquid polymers are known. These methods use an electrode defining an orifice. The liquid is passed through the orifice, from a first side of the electrode to a second side. An oppositely charged surface is remotely disposed with respect to the electrode, on the second side of the electrode, to attract and collect the fibers formed after the fluid issues from the orifice. These methods require large potential differences developed over the large air gap between the orifice and the charged surface on which the fibers are collected. The electric field developed over the air gap is relied upon to develop the necessary charge within the fluid and attenuate the fluid. The attenuated fluid then solidifies into fibers. For low conductivity fluids, such as liquid polymers utilized to develop fibers for commercial applications such as fabrics, the flow rates attained by these methods are unacceptable. Known methods also include the use of a capillary needle as the electrode and orifice discussed above. Fibers having diameters of 50 nanometers and up have been produced utilizing these methods.
Electrostatic formation of fibers has great potential and it has been known that electrostatic formation of fibers would present a much more convenient and efficient method of producing fibers. However, despite considerable effort to develop these methods, these methods have been unable to handle commercially acceptable flow rates.
The present invention addresses these needs.
In accordance with one aspect of the present invention, a method of producing fibers comprises providing a stream of a solidifiable fluid, providing the stream with a net charge so as to disrupt the stream by passing the stream through a body defining an orifice so that the stream passes through an electric field before exiting the orifice, and allowing the disrupted stream to solidify to form fibers. xe2x80x9cSolidifyxe2x80x9d as used herein, means a marked change in viscosity or change in state such that the material tends to retain a definite shape. xe2x80x9cSolidifyxe2x80x9d as used herein includes a change in the fluid to an elastomeric fiber, rigid or semi-rigid fibers, and solid or semi-solid fibers.
Preferably, the step of providing the stream with a net charge includes injecting a net charge into the stream. The step of injecting a net charge preferably includes injecting a net charge so as to develop a self electric field for the stream of at least 0.5 megavolts per meter. Charge injection of the solidifiable fluid achieves a high charge density in the fluid. Charge injection creates a strong xe2x80x9cself-fieldxe2x80x9d within and in proximity to the fluid stream, and the fluid stream forms fibers under the influence of the self-field.
In certain preferred embodiments, a pair of electrodes is provided in the vicinity of the orifice while a potential difference is maintained between the electrodes. One of the pair of electrodes may comprise the body. An electric field is developed between the electrode and the body so that the stream is provided with a net charge. Charge injection occurs within the stream of fluid, in the space between the electrode and the body defining the orifice.
The self-field within and immediately surrounding the fluid stream causes the fluid stream to break into highly elongated filaments which solidify to form solid fibers. A further surface remote from the orifice such as a container or a collection reel may be used to collect the fibers. This surface may be at the same potential as the body defining the orifice, or may be at a different potential. However, there is no need to provide a large potential difference between this surface and the body. Typically, both the body defining the orifice and the collecting surface are grounded.
The limit on the flow rate of the solidifiable fluid is the size of the orifice so that throughput orders of magnitude greater than the known electrostatic methods is achieved. The improved throughput is surprising. Embodiments in accordance with the invention have achieved throughputs great enough for industrial production of fibers.
The method, in certain preferred embodiments, comprises heating the disrupted stream as it passes out of the orifice. The step of providing the stream with a net charge preferably provides the stream with a charge density of at least 0.5 coulombs per cubic meter.
The step of injecting a net charge, in certain preferred embodiments, comprises passing the stream past an electron gun located adjacent the orifice.
The step of providing a stream of a solidifiable fluid may comprise passing the solidifiable fluid through an orifice at a rate of at least 0.1 grams per second, in certain embodiments, or a rate of at least 0.5 grams per second, in other embodiments. The solidifiable fluid may be passed through an orifice at a rate of at least 1 gram per second.
The step of providing a stream of solidifiable fluid may include heating a polymeric material and the step of allowing the stream to solidify may comprise allowing the disrupted stream to cool. The step of providing a stream of a solidifiable fluid may comprise providing a polymeric material in a solvent and the step of allowing the stream to solidify may comprise allowing the solvent to evaporate.
The solidifiable fluid may comprise a liquid polymer, for example. In certain preferred embodiments, the liquid polymer comprises a molten polymer.
The solidifiable fluid may comprise a liquid glass, a liquid polyester, such as polytetrafluoroethylene, polyethylene terephthalate (xe2x80x9cPETxe2x80x9d), polybutylene terephthalate, or a liquid thermoplastic polyurethane.
The solidifiable fluid may comprise a liquid solution including a polymeric material, such as LEXAN(copyright) and methylene chloride, or tetrahydrofurane and urethane.
Another aspect of the present invention, is an electrostatically formed fiber produced by the providing a stream of a solidifiable fluid, providing the stream with a net charge so as to disrupt the stream by passing the stream through a body defining an orifice so that the stream passes through an electric field prior to exiting the orifice, and allowing the disrupted stream to solidify to form fibers. The fiber may be formed of a polyester, a polytetra fluoroethylene, polyethylene terephthalate, polybutylene terephthalate, thermoplastic polyurethane, carbon, or glass. The fibers preferably have a diameter of less than 100 micrometers, more preferably less than 10 micrometers. In certain preferred embodiments, the fiber has a diameter of less than 500 nanometers, preferably less than 100 nanometers, even more preferably less than 20 nanometers.
In another aspect of the present invention, a method of producing fibers comprises providing a plurality of streams of solidifiable fluid. Each of the plurality of streams is provided with a net charge so as to disrupt the streams by passing each stream through a structure defining an orifice so that the stream passes through an electric field prior to exiting the orifice. Each disrupted stream is allowed to solidify to form fibers. Orifices for multiple streams may be utilized in an assembly for generating fibers on a large scale.
In another aspect of the present invention, a method of forming a charged solid comprises providing a stream of a solidifiable fluid, providing the stream with a net charge by passing the stream through a body defining an orifice so that the stream passes through an electric field prior to exiting the orifice, and allowing the stream of solidifiable fluid to solidify while still charged. In certain preferred embodiments, the stream disrupts under the influence of the net charge. Preferably, the stream of solidifiable fluid has a maximum charge mobility of 10xe2x88x926 m2/Vxe2x80xa2sec. Preferably, the stream of solidifiable fluid has a minimum net charge of 0.1 coulombs per cubic meter.
In yet another aspect of the present invention, an apparatus for producing fibers comprises a feed system adapted to deliver a stream of molten polymeric material, and a charge injection device adapted to provide the stream with a net charge so as to disrupt the stream, said device comprising a body defining an orifice and being arranged so that the stream passes through an electric field prior to exiting the orifice.
The feed system preferably comprises at least one heater for melting the polymeric material. In certain preferred embodiments, the charge injection device comprises a pair of electrodes, in which one of the pair of electrodes comprises the body defining the orifice. In other embodiments, the charge injection device comprises an electron gun.
In another aspect of the present invention, a method of forming fibers comprises providing a stream of a solidifiable fluid at a rate of at least about 0.02 grams per second, injecting electrical charge into the stream of solidifiable fluid, whereby the stream will tend to disperse and form filaments, and solidifying the filaments. The method preferably comprises injecting electrical charge so as to inject at least about 1 coulomb per cubic meter. The method preferably comprises providing the stream of fluid at a rate of at least 0.1 gram per second and more preferably at least 1 gram per second.
In another aspect of the present invention, a method of forming fibers comprises providing a stream of a solidifiable fluid, injecting at least about 1 coulomb of electrical charge per cubic meter of fluid into the stream of solidifiable fluid, whereby the stream will tend to disperse and form filaments and solidifying the filaments. Preferably, the stream is provided at a rate of at least about 0.02 grams per second.
In another aspect of the present invention, a method of forming fibers comprises providing a stream of a solidifiable fluid at a rate of at least about 0.03 milliliters per second, injecting electrical charge into the stream of solidifiable fluid, whereby the stream will tend to disperse and form filaments, and solidifying the filaments. The method preferably comprises injecting electrical charge so as to inject at least about 1 coulomb per cubic meter into the solidifiable fluid. The method preferably comprises providing the stream of fluid at a rate of at least 0.1 gram per second and more preferably at least 1 gram per second.